vip, vi and vi_model S3 methods for workflow objects

[Athospd]

vip, vi and vi_model S3 methods for workflow objects

ref: https://tidymodels.github.io/workflows/

[bgreenwell]

Thanks for the PR @Athospd, any chance you can add some tests?

Example here: https://github.com/koalaverse/vip/blob/master/inst/tinytest/test_pkg_parsnip.R

[Athospd]

Of course! I looked for the tests in the wrong folder =P (there is a empty 'tests' folder btw).

[topepo]

Can I suggest that adding a function to check to see if the workflow has been fit (right before passing the fit object in)?

check_wflow_fittted <- function(x) {
  if (is.null(x$fit$fit)) {
    stop("Please fit the workflow.")
  }
  invisible(x)
}

[bgreenwell]

Updated and addressed in #128.